This invention relates to photographic processing, and more specifically to forced development of multilayer emulsions.
It has been known for years that underexposed silver halide emulsion can be "forced" by overdevelopment to yield a recognizable image when normal development would yield a non-recognizable image. The forcing is accomplished by increasing the temperature of the developing solution above normal, or by increasing the time interval of development, or both.
However, it has been equally well known that the contrast of the forced development image has been excessive, as has been the "grain" of the image. That is, the excessive size of the silver particle groups that give a spurious pattern over the image, having the appearance of a black snow storm.
Further, such forced development has engendered "fog", the known uniform spurious overlay upon the image that masks faint details.
Another disability enters when plural-layer color film is force developed. In the rendition of a color image it is necessary that the straight-line "curves" of a graph of density vs. exposure relation of each of the color components lie in parallel relationships. This is accomplished in normal development.
However, when the development is forced according to the prior art as previously described it is common for two of the curves to intersect at high densities; say the red and the green color components. In the resulting positive print the flesh tones will have a red tone and/or the shadows will have a green tone. Such a condition cannot be remedied. The art describes this situation by saying "the curves no longer `track`".
At times in the past a "two-part developer" has been used. In such processing the film emulsions are first brought into contact with a first part of the developer. This part has had essentially all of the ingredients for development except for an ingredient that will allow the developer to develop the silver halide. The second part contains only one, or only a few, ingredients, such as only the hydroxyl radical OH.
Immersion of the emulsion in a solution having that ingredient causes development to take place. When the second solution becomes a conventional developer it must be discarded.
It is known that such a process of developing does not result in a significant increase of sensitivity ("speed") of the emulsion(s) involved.
Prior art forcing techniques have often employed "flashing"; i.e. flooding the exposed but not yet developed negative with a weak light in an attempt to correct for forced processing difficulties.